1. Field of the Invention
The present general inventive concept relates to an illuminating unit to emit a collimated beam and an image projection system including the same.
2. Description of the Related Art
An illuminating unit employs various types of collimators including lenses to efficiently collimate a beam from a light source to a beam at a radiation angle to effectively illuminate an object.
For example, an image projection system may include an illuminating unit illuminating a light modulator. When a large, bulky metal halide lamp or an ultrahigh-pressure mercury lamp is used as a light source in the illuminating unit, the illuminating unit becomes bulky. Furthermore, the lamp has a short life span lasting only up to several thousand hours and requires frequent replacement when it is used at home. To solve this problem, research into use of a compact light source such as a longer-lasting light emitting diode (LED) is currently being conducted.
Since an LED typically has a smaller amount of light than the metal halide or ultrahigh-pressure mercury lamp, an LED array of a plurality of LEDs is used as a light source in an image projection system. In this case, a lens is typically used to condense light emitted by the LED array. However, use of the lens degrades optical efficiency.
The degradation of optical efficiency will now be described in more detail with reference to FIGS. 1 and 2. The product of the size and angle of an image at a paraxial region is conserved. Thus, the product of a light-emitting area of an LED multiplied by a solid angle of a light emitting angle becomes the amount of conservation, which is referred to as “etendue.” As shown in FIG. 1, when a single LED is used in a conventional image projection system, the product of a light-emitting area φL of the LED multiplied by a solid angle UL is made equal to the product of the area φP and a solid angle UP of an optical modulator.
When an array of a plurality of LEDs are used in a conventional image projection system as shown in FIG. 2, the sum ΣφL of light-emitting areas for the LED array is greater than the light-emitting area φL of the single LED while a solid angle UL of a light emitting angle of the LED array and the area φP of a light modulator are equal to their counterparts for the single LED. Thus, to conserve etendue, a solid angle UP′ of a light emitting angle of an optical modulator increases compared to when a single LED is used. Since the solid angle UP′ is greater than an angle UP that light can effectively be projected by a projection lens, light in a range of an angle greater the angle UP cannot be effectively projected by the projection lens. This results in optical losses, thereby degrading optical efficiency. Consequently, while having the increased number of LEDs, the brightness of a conventional image projection system can only be increased to a limited degree.